Television receiver with automatic shade-level control



Oct. 21,1941. H. A. WHEELER 2,259,538

TELEVISION RECEIVER WITH AUTOMATIC SHADELEVEL CONTROL Filed 96. s, 1938INVENTOR AROLD AJNHEELER ATTORNEY Patented Oct. 21,1941

TELEVISION RECEIVER WITH AUTOMATIC SHADE-LEVEL CONTROL Harold A.Wheeler, Great Neck, N. 1., a-ignor to Haz'eltlne Corporation, acorporation of Dela- Application December 8, 1938, Serial No. 244,190

Claims.

This invention relates to television receiving apparatus and moreparticularly to automatic control systems for such apparatus. Theinvention is especially directed to the provision of an improved signalshade-level control system for television receivers, which is useful asa blacklevel adjusting device.

In accordance with present television practice, a transmitted signalcomprises a carrier, modulated during successive intervals or traceperiods by video frequency, and steady components representative oflight variations in an image being transmitted and of its averagebackground illumination, respectively. Between the trace periods, thatis, during retrace intervals, the carrier has a predetermined amplitudelevel, referred to as a blanking level, corresponding to a given shade,usually black, and is modulated during a part of this retrace period bysynchronizing components which correspond to initiations of successivelines and fields in the scanning of the image.

At the receiver, a beam is so deflected as to scan and illuminate atarget or screen in a series of fields of parallel lines. Thesynchronizing components of the received signal are separated from theother modulation-signal components and utilized to control the scanningapparatus of the receiver so as to synchronize its operation with thatof similar apparatus utilized at the transmitter in developing thesignal. The intensity of the beam is controlled by the lightmodulationcomponents, thereby to reconstruct the image.

An important requirement in a properly operating television receiver isthe adjustment of the system so that the modulation signal is stabilizedwithrespect to a particular shade value or level, such as the blankinglevel. More particularly, the system must be so controlled-that a givensignal level or amplitude corresponding to black or any predeterminedshade value is effectively held fixed with respect to the signal-inputbrightness characteristic of the reproducing device of the system at theinput circuit of this device, in order that signal componentsrepresenting any given shade will at all times appear as the same shadein the reproduced image. In general, the system should be controlled sothat the light gradations or shades from black to white represented bythe various light-modulation components are all faithfully representedin the reconstructed image.

fore, to provide an improved signal shade-level adjusting system for atelevision receiver.

In accordance with the present invention, a

television receiver adapted to receive a signal containing synchronizingcomponents and video-frequency components comprises apicture-reproducing device, means for deriving from the received signaland applying to the device a signal having a periodically recurringlevel corresponding to a predetermined shade value, and means connectedin circuit with the device for developing a voltage varying with thecurrent therein. The receiver also includes an auxiliary circuit, meansfor applying the above-mentioned developed voltage to the auxiliarycircuit, means responsive to the synchronizing components for disablingthe voltage-developing means whenever the current in thepicture-reproducing device exceeds a predetermined value correspondingto the above-mentioned predetermined shade value, and means includingthe auxiliary circuit for superimposing on the signal applied to saidpicture-reproducing device a bias for maintaining the current in thedevice at a predetermined value upon the occurrence of said recurringlevel in said signal.

For a better understanding of the present invention, together with otherand further objects thereof, reference is bad to the followingdescription taken in connection with the accompanying drawing,.and itsscope will be pointed out in the appended claims.

In the accompanying drawing, Fig. 1 is a circuit diagram, partially inblock form, of a cathode-ray tube television receiving system includingcircuits embodying the present invention; Fig. 2 is a graph of the.envelope of a negatively modulated television carrier wave to aid inthe understanding of the invention; w hile Figs. 3-5, inclusive, aregroups of wave forms representing the derived signal developed atdifierent points in the receiver of Fig. 1.

Referring now more particularly to Fig. 1 of the drawing, the systemthere illustrated comprises a receiver of the superheterodyne typeincluding an antenna system lllll connected to a radio-frequencyamplifier II, to which there are connected in cascade. in the ordernamed, an oscillatormodulator IS, an intermediate-frequency amplifier I,a detector IS, a video-frequency amplifler l8, and a cathode-raysignal-reproducing tube ll.

The cathode-ray tube ll comprises the usual envelope l8 containing, inthe order named, a

It is an object of the present invention, therecathode l8 having aheater 20, a signal-input or control 2|, a screen 22, a first anode 23,a second anode 24, and the usual fluorescent screen 25 at the end of thetube- Line-frequency and field-frequency scanning coils 28 and 21 aredisposed about the tube for defiectingthe beam in The out two directionsnormal to each other. put circuit of the video-frequency amplifier isconnected to the control grid 2| by way 01' a coupling condenser 23 andresistor 29. Operating potentials for the several electrodes of thedevice included in the cathode circuit of the tube a l I are suppliedfrom a suitable source, such as the battery 30; v

A line-frequency generator 3| and a field-frequency generator 32 arealso coupled to the output circuit of the detector l5 forsynchronization and are connected to the scanning coils 26 and 21,respectively, of the cathode-ray tube in the conventional manner.Suitable means are included in the generators 3| and 32 for selectingthe synchronizing pulses from the other modulation components and fromeach other. The stages or units just described may all be ofconventional well-known construction so that detailed illustrations anddescriptions thereof are unnecessary herein.

Referring briefly to the general operation of the system described aboveas a whole, television signals intercepted by the antenna circuit ||i-||are selected and amplified in the radio-frequency amplifier i2 andsupplied to the oscillator-modulator 3, wherein they are converted tointermediate-frequency signals which, in turn, are selectively amplifiedin the intermediate-frequency amplifier l4 and delivered to the detectorii. The modulation componentsof the signal are derived by the detector.l5 and are supplied to the video-frequency amplifier l6 wherein theyare amplified and from which they are supplied to the control grid. 2|of the cathode-ray tube ll in a manner hereinafter explained in detail,which is in accordance with the present invention.

The detected signal components are also supplied to the generators 3|and 32 to synchronize the operation thereof. With properoperatingpotentials supplied from the battery 30 to the electrodes of the tubeii, an electron beam is emitted from the cathode. l9 and its intensityis controlled by the grid 2| in accordance'with the video-frequencyvoltages and control voltages impressed thereon. The screen 22 serves tocontrolthe average beam intensity while the first and second anodes 23and 24 cooperate to accelerate and focus the beam. It will beappreciated that the tube should have a substantially linear response orsignal-input brightness characteristic, as distinguished from the curvedcharacteristics of conventional amplifier tubes, so that variations inits grid-bias voltage serve simplyto adjust the point on thischaracteristic at which the tube operates, substantially withoutaffecting its gain, so as to determine the average brightness of thereproduced image for a given input-signal level.

The intensity of the electron beam of the tube i1 is modulated orcontrolled in accordance with the video-frequency voltages impressedupon the control grid of the tube in the usual manner. Saw-tooth currentwaves are generated in the line-frequency and field-frequency generators3| and 32, which are controlled by the detected synchronizing pulses,and these waves are applied to the scanning elements of the cathode-raytube I! to produce electric scanning fields, thereby to defiect the rayin two directions normal to each other, so as to trace a rectilinearscannin natvacuum-tube conductance 33, for example, a triode having itscontrol grid connected to a suitable point in the generator 3|, wherebynegative pulses are applied to this grid during'the lineretracescanningintervals. Such waves are developed and conveniently availablein all conven-' tional scanninggenerators. This grid connection may, ifdesired, be to the field-frequency generator 32, instead of thegenerator 3|, for applying negative pulses to the tube 33 during thefieldretrace periods, or connections may be made to both generators forapplying both line-retrace and field-retrace pulses. In any of thesecases, the principle of operation of the system is exactly the same aswill be readily appreciated from the following description. The tube 33is normally conductive and is rendered non-conductive by the appliednegative pulses during the entire retrace period including intervalsduring which the signal amplitude corresponds to the blanking level. Arectifier circuit, including a diode 34 in series with aparallel-connected resistor 35 and condenser 36 having a predeterminedtime constant, which is much greater than the period of thelinesynchronizing intervals, is connected across the tube 33 fordeveloping a unidirectional-bias voltage varying in accordance with thecurrent passed by the cathode circuit of the cathode ray tube I1 duringthe line-retrace periods when the tube 33 is non-conductive; that is,varying in accordance with the blanking level.

For applying the unidirectional voltage developed by the diode 34 andits load circuit to the control grid of the cathode-ray tube withgreater amplitude and reversed polarity, there is provided an auxiliaryor bridge circuit including a resistance network, one arm of whichcomprises a source of unidirectional voltage, for example, a battery 31,grounded at an intermediate terminal, and the other arm of whichcomprises a resistor 38 connected across battery 31. A repeater or aunilaterally-conductive device comprising, for example, a vacuum tube 39is connected between points of the network normally effective to producea current therethrough; specifically, it is connected between anintermediate point on the resistor 38 and an intermediate point ofpositive potential on the battery 31. A second intermediate point on theresistor 38 is connected by way of the resistor 29 to the control grid2| 0: the

cathode-ray tube. The control grid of the tube 33 is connected to thepositive end of the diode load resistor 35, thereby to apply the voltagedeveloped across resistor 35 to the auxiliary circuit through theunilaterally-conductive device 39 to effect, in at least a portion ofthe above-mentioned resistance network, a variation in theunidirectional current therein to produce, in at least a part of thenetwork, a current in opposition to the first current and varying inaccordance with variations in the level of the received signal from apredetermined level, specifically, the blanking level.

The fixed bias provided for the tube 33 by virtue of its connection inthe bridge circuit is such that it always passes current and the amountof this.

current is dependent upon the positive potential applied to its grid bythe diode circuit 34, 3|, .0. The potentials at the intermediate pointson the resistor 38, where the anode of the tube II is connected andwhere the control grid of the cathode-ray tube I1 is connected, thusvary in blanking level, as will be clear from the descrip tion of theoperation 01' the system in connection with Figs. 2-5, inclusive, whichiollows.

For the purpose of effectively stabilizing the modulation signal withrespect to the peaks ,of the synchronizing pulses, there is provided adiode 40 connected in parallel with the resistor 29, as

shown, which operates to rectiiy the peaks of the signal voltage outputoi! the video-frequency amplifier ll, thereby to develop a positiveunidirectional .voltage equal to the peak value of the unstabilizedsignal in the black direction. This voltage appears across the resistor28 and is applied positively to the cathode-ray tube control grid 2| inopposition to the other voltage applied thereto by way of this resistorand thus, in effect, so adjusts the axis or the signal wave that thepeaks of the synchronizing pulses are substantially aligned.

The operation and results obtained by the system of Fig. 1 may best beunderstood by reference to Figs. 2-5, inclusive. In Fig. 2 there areillustrated the wave forms of portions of the complete carrier envelopesof television modulated-carrier waves of the negatively modulated type.The portion shown at A represents a wave of relatively large amplitudewhile that at B represents a similar wave difiering only in that it isof lesser amplitude, as might be caused by occasional i'ading. In Figs.3-5 corresponding wave portions are shown, illustrating the form of themodulation signal derived at various points oi the system for theassumed signal inputs of different amplitudes A and B.

The portions oi the wave of Fig. 2 indicated at L indicate theline-retrace periods and the level.

indicated at Bl represents the periodically recurring blanking levelcorresponding to black or some other predetermined shade value. Theportion of the wave at which the carrier amplitude is greatest duringthe retrace periods, certain of which are indicated at 8, representsline-synchronizing pulses. The portions between the retrace periodsrepresent trace portions of successive lines, certain of these beingmodulated, as indicated at M1 and m, by the high-frequency andbackground-light components. During the line-retrace and field-retraceperiods, part of one of the latter being indicated at Y-Y, the amplitudeof the wave is independent oi light-modulation components. Thefield-retrace periods ordinarily include the line-synchronizing pulses,as well as additional broad pulses, not shown, which latter constitutecollectively a field-synchronizing in accordance with the low-frequencyor steady background or average illumination components oi the image.Thus, in the particular portions of the wave shown, during theline-trace modulation periods represented at M1, the amplitude Y of thecarrier is such that it reaches the white level, whereas this does notoccur during the modulation periods represented at Ma. Since the averageintensity of the carrier varies in accordance withbackground-illumination variations, as well as iading, different fieldstrengths oi the signal, and the like, there may be no fixed field levelthroughout the signal which is representative of a given shade value.However, the blanking level, at which the signal may represent black or"blacker than black" and which appears in the signal-not only for black-pictures but also during the field-retrace periods. as indicated at Blin Fig. 2, has different values for the different signal intensities inthe signal portions A and B and is independent of light-modulationcomponents. This blanking level, therefore, aiiords a measure .01 theintensity of the carrier wave which may be utilized automatically tocontrol certain characteristics oi the receiver for the purpose ofsetting, or stabilizing with respect to, the black level in the signalto be reproduced.

In the operation of the present invention, therefore, themodulated-carrier wave is detected by the detector IE to derive avoltage wave corresponding to one-half the modulation envelope of Fig.2, this voltage being impressed on the input circuit 01, and amplifiedin, the amplifier l 8. The

steady component of the modulation signal is lost when the signal passesthrough the condenser 28 so that it centers itself about its zero axisand tends to have a wave. form such as is illustrated by the curves ofFig. 3. The diode 40 derives from the signal output of the amplifier Ita unidirectional voltage equal to the peak voltage equal to the peakvalue of the signal on the black side 01' the zero axis, which voltageappears across the resistor 29 and is applied, as previously explained,positively to the control grid of the oathode-ray tube in opposition tothe bias voltage applied thereto from the bridge circuit 31, 38 by wayof the resistor 29. This positive voltage serves automatically to varythe bias of the constantially a direct one, sothat the unidirectionalcomponent of the signal is not lost, the signal may be sufficientlystabilized with respect to the synchronizing pulses without the use ofthe stabilizing tube 40. However, where a non-conductive couplingintervenes between the detector l5 and the control grid circuit, such asthe condenser 28, the steady component is suppressed and the' modulationsignal tends to center itself about the zero axis and, as stated above,to appear as il lustrated by the curvesof Fig. 3. In this case,therefore, it should be stabilized as explained.

It will be seen from the curves of Fig. 4 that, even with the signalstabilized with respect to the peaks of its synchronizing pulses, if, byreason of fading or the like, the average signal intensity independentof light modulation varies the level or amplitude in the signal whichrepresents black or any given shade value correspondingly varies, sothat signal components representing the same shade value have differentamplitudes depending on the signal intensity variation.

The control circuit of the present invention operates to correct thiscondition. More particularly, the cathode of the cathode-ray tube I8 isnormally, during trace periods, connected to ground by way of the tube33 and the grid bias of the cathode-ray tube is originally adjusted, bymeans of the location of the grid lead on the resistor 38, so as tocorrespond to dull gray; that is, to provide a beam current suflicientto cause this shade to be reproduced by the tube. negative pulsesapplied to the control grid of the tube 33 from the line-frequencygenerator 3| render the tube 33 nonconductive only for the durations ofthe line-retrace periods, and during the remaining intervals, that is,while the current in picture-reproducing tube l'I exceeds the blankingvalue, it effectively by-passes or disables therectifier circuit 34, 35,36. The circuit of the diode 34 is energized, therefore, only when theseretrace pulses are applied to the tube 33, and during these periods therectifier circuit 34, 35, 36 conducts the small cathode current,corresponding to gray. The value of the cathode current of the tube I1during the retrace periods is dependent upon the blanking level of thesignal which, as may be seen from Fig. 4, is higher for signals ofgreater intensity. In other words, the

diode 34 and its load circuit are connected to the picture-reproducingdevice I! to develop a voltage varying with the current therein andserve to develop a bias voltage varying with the blanking level duringthe retrace periods and apply it positively to the control grid of thetube 39 which, with its associated auxiliary bridge circuit, operates asa direct current reversing amplifier. The space current of the tube 39flows through the resistor 38 so that the potentials of the intermediatepoints thereof vary oppositely with variations of the biasvoltage outputof rectifier 34, 35, 36. Therefore, the unidirectional voltages at thepoint on the resistor 38 where the anode of the tube 39 is connectedand, hence, at the point where the lead to the control grid of thecathode-ray tube I1 is connected, shift negatively in accordance withany increase in the amplitude of the blanking or black level of thesignal. This bias voltage is or proper polarity to provide a variablebias on the control grid 2i which serves to adjust the point ofoperation of the signal input on the response characteristic of the tubel1 and thereby maintain the signal level corresponding to blacksubstantially at the cutoff point 01' the tube characteristic at alltimes, that is, to maintain the current in tube II at the predeterminedvalue corresponding to the black shade level upon the occurrence of therecurring blanking level in the signal.

Thus, the signal as effectively applied to the control grid 2| has awave form such as illustrated in Fig. 5, where, it will be seen, theblanking level BI is the same regardless of variations in the signalintensity. Preferably this level is, as stated above, at the cutoffpoint on the inputsignal'brightness characteristic of the reproducingtube. In other words, the net result of the control is to reduce thecathode current of the cathode-ray tube nearly to zero for a signallevel corresponding to black. While it cannot be reduced entirely tozero since the operation of the The.

control requires some cathode current, itdoes, however, closely approachthe ideal of having the signal level corresponding ,to blacksubstantially at the cutoff point. Thus the image is recon-' ance withthe present invention.

It is to be noted that, while the invention has been described above inconnection with a receiver adapted for a negatively modulated signal,the invention is equally applicable in connection with a receiveradapted for a positively modulated signal, as will be readilyappreciated by those skilled in the art.

While there has been described what is at present considered to be thepreferred embodiment of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,aimed in the appended claims to cover all such changes and modificationsas fall within the true spirit'and scope of the invention.

What is claimed is:

1. A television receiver adapted to receive a signal containingsynchronizing components and video-frequency components comprising, apicture-reproducing device, means for deriving from the received signaland applying to said device a signal having a periodically recurringlevel corresponding to a predetermined shade value, means connected inthe circuit with said device for developing a voltage varying with thecurrent therein, an auxiliary circuit, means for applying said developedvoltage to said auxiliary circuit, means responsive to saidsynchronizing components for disabling said voltage-developing meanswhenever the current in said device exceeds a predetermined valuecorresponding to said predetermined shade value, and means includingsaid auxiliary circuit for superimposing on said signal applied to saiddevice a bias for maintaining the current-in said device at saidpredetermined value upon the occurrence of said recurring level in saidsignal.

2. A television receiver adapted to receive a sig- 'nal containingsynchronizing components and video-frequency components comprising, apicture-reproducing device, means for deriving from the received signaland applying to said device a signal having a periodically recurringlevel corresponding to the value of black, means'connected in thecircuit with said device for developing a voltage varying with thecurrent therein, an auxiliary circuit, means for applying said developedvoltage to said auxiliary circuit, means responsive to saidsynchronizing components for disabling said voltage-developing meanswhenever the current in said device exceeds a predetermined valuecorresponding to said black value, and means including said auxiliarycircuit for superimposing on said signal applied to said device a biasfor maintainng the current in said device at said predetermined valueupon the occurrence of said recurring level in said signal.

3. A television receiver adapted to receive a modulation signalcontaining synchronizing components and video -frequency componentscomprising, a picture-reproducing device, means for deriving from thereceived signal and applying to said device a signal having aperiodically recurring level corresponding to a predetermined shadevalue, means connected in circuit with said device for developing avoltage varying with the current therein, an auxiliary circuit includinga resistance network, means for producing a unidirectional current insaidnetwork and a unilaterally-conductive device coupled between pointsof said network normally effective to produce a current therethrough,means for applying said developed voltage to said auxiliary circuitthrough said unilaterallyconductive device toeflfect in at least aportion 1 of said network a variation in said unidirectional current toproduce in at least a part oi said network a current in opposition tosaid first current and varying in accordance with variations in saidlevel of said signal from a predetermined level, means responsive tosaid synchronizing components or rendering said unilaterally-conductivedevice nonconductive whenever the current in said device should exceed apredetermined value co responding to said shade value, andvoltage-utilization means for controlling'said device to stabilize saidmodulation signal with respect to said level and connected to a point onsaid network the potential of which is varied by said opposing currentin accordance with the amplitude thereoi'.

4. A television receiver adapted to receive a signal containingsynchronizing components and video-frequency components comprising, apicture-reproducing device, means for deriving from the received signaland for applying to said device a signal having a periodically recurringlevel corresponding to a predetermined shade value, means connected incircuit with said device i'or developing a voltage varying with thecurrent therein, an auxiliary circuit including a bridge circuit thearms of which comprise a source or unidirectional voltage and a resistorrent therethrough in accordance with variations through which currentnormally flows, a vacuum- 0 in said level of said signal to produce inat least a part of the resistance arm oi said bridge a current inopposition to said first current and proportioned to variations of saidlevel, means responsive to said synchronizing components for disablingsaid last-mentioned means whenever the current in said device exceeds apredetermined value corresponding to said shade value, and meansincluding said auxiliary circuit for controlling said device tostabilize said modulation signal with respect to said level and con-'nected to a point on said resistor the potential of which is varied bysaid opposing current in accordance with the amplitude thereof.

5. A television receiver adapted to receive a signal containingsynchronizing components and video-frequency components comprising apicture-reproducing device having a predetermined cutoff characteristic,means for deriving from the received signal and applying to said devicea signal having a periodically recurring level corresponding to apredetermined shade value, means connected in the circuit with saiddevice for developing a voltage varying with the current therein, anauxiliary circuit, means for applying said developed voltage to saidauxiliary circuit,

'means responsive to said synchronizing compo- HAROLD A. WEEEIER.

